The Pope has apologised for the "unspeakable crimes" committed by Catholic priests who sexually abused children.
In the most strongly worded public apology of his state visit so far, the Pontiff spoke of the "shame and humiliation" brought by the scandal.
He told the congregation at London's Westminster Cathedral: "I think of the immense suffering caused by the abuse of children, especially within the Church and by her ministers.
"Above all, I express my deep sorrow to the innocent victims of these unspeakable crimes, along with my hope that the power of Christ's grace, his sacrifice of reconciliation, will bring deep healing and peace to their lives.
"I also acknowledge with you the shame and humiliation which all of us have suffered because of these sins."
The apology comes after he acknowledged this week that the Catholic Church had failed to deal with abusive priests decisively or quickly enough.
Earlier, the Pontiff was greeted by Prime Minster David Cameron - the first time the two men have met since the Pope arrived in the UK.
The religious leader also spoke with other political figures including Deputy prime minister Nick Clegg and acting opposition leader Harriet Harman.
Pope Benedict XVI's address comes as six men remain in police custody after they were arrested over an alleged plot to attack him.
Five street cleaners were arrested on Friday morning as armed officers swooped on a depot as they prepared to start their shift. Hours later a sixth man was arrested at a home in north London.
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